The present invention generally relates to a spot welding. More specifically, the invention relates to a spot welding system and a spot welding method in which an estimation is made of a size of "a weld zone once melted upon a current conduction at a welding spot under an exerted welding pressure, and solidified thereafter" (hereafter called "nugget") and an estimated nugget size is employed as a base for a control on a quality of a weld at the spot, as well as to a nugget diameter estimation system and a nugget diameter estimation method applicable to such a spot welding system and a spot welding method.
FIG. 1 illustrates a typical nugget in a section of a welding spot.
Two base metal sheets 101, 102, lapped either on the other, are held with forces between stationary and movable electrode 104, and subjected to a welding current conducted therethrough, whereby they 101, 102 are heated, and start swelling in bulk, while melting from a central region of a welding spot W.
In cases such as of an excessive welding current, a deficient holding force and/or an insufficient preparation of a base metal surface, a body of molten metal may spatter outside or between the base metal sheets 101, 102, forming a surface flash 106 or as an expulsion 105, respectively, having a blow hole 107 left in a nugget N.
A finish of current conduction is followed by a contraction of the base metal sheets 101, 102 that gives final dimensions of a size of the nugget N, e.g. an outside diameter Nd and a thickness Nt thereof. The nugget diameter Nd may be deemed as an inside diameter of a corona bond 108 that is a pressure welded portion around the nugget N. After the contraction, the base metal sheets 101, 102 may have a slight sheet separation 109 incurred therebetween or an indentation 110 left as a trace of pressed electrode on a base metal surface. Non-conforming welding conditions may cause a pickup 111 or pit 112.
It is known to observe such developments in growth of nuggets for utilization to a quality control of a spot welding.
U.S. Pat. No. 4,542,277 discloses an apparatus for observing developments in growth of nuggets in terms of a displacement of an electrode to base a decision of a welding quality on a maximum displacement of the electrode, while suggesting a general usefulness of a computer processing of data on developments in growth of nuggets.
Japanese Patent Publication No. 48-41422 and Japanese Patent Publication No. 53-4057 compare a maximum variation of an inter-electrode distance with a reference variation, for a decision on a sureness of a weld at a welding spot.
Further, the Japanese Patent Publication No. 53-4057 as well as U.S. Pat. No. 3,400,242 compares a varying rate of an inter-electrode distance due to an expansion of a welding spot within a predetermined time interval, i.e. an expanding rate, with a reference varying rate for a decision on a welding sureness.
Japanese Patent Application Laid-Open Publication No. 7-232279 compares a time for a decrement of an inter-electrode distance due to a contraction of a welding spot, i.e. a contracting rate, with a reference time for a decision on a welding sureness.
The use of an instantaneous value, such as a maximum displacement or maximum variation, observed as a criterion for a decision permits the decision to be efficiently performed at a level where one can neglect developments of nugget growth before and after a point of time for the observation.
The use of a so-called `differential value` within a predetermined time interval, such as an expanding rate or contracting rate, as a criterion for a decision permits the decision to be efficiently performed at a level allowing for considerations to a linear development of nugget growth within an associated observation time.
However, for decisions to be higher in accuracy and/or quick in response, e.g. for those in a robot line for spot-welding automobile bodies, their criteria sometimes need a level at which a non-linear development of nugget growth should also be taken into consideration.